Ductile low temperature brazing alloy

ABSTRACT

Reactive metal silver alloys containing controlled levels of copper, aluminum or mixtures thereof liquidus temperatures in the range of from about 750° C. to about 950° C., are ductile and after brazing are relatively free of hard dispersed phases. The level of reactive metal is kept below 4% by weight and from about 0.25% to about 2.5% is preferred.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of Ser. No. 877,628, filed June 23, 1986, now U.S. Pat. No. 4,684,579 which was a continuation-in-part of Ser. No. 823,200 filed Jan. 28, 1986 now abandoned which was a continuation of Ser. No. 757,962 filed Sept. 20, 1984 now U.S. Pat. No. 4,623,513 which was a continuation of Ser. No. 394,226 filed on July 1, 1982, now abandoned.

FIELD OF INVENTION

This invention relates to brazing alloys. More particularly it relates to low temperature brazing alloys containing titanium.

BACKGROUND

An alloy foil sold under the trademark of Ticusil by the Wesgo Division of GTE Products Corporation, Belmont, Calif. contains 4.5% by weight of titanium, 68.8% by weight of silver and 26.7% by weight of copper. This material is sold as a composite because an alloy having these elements, while having the ability to wet various ceramic materials and a relatively low brazing temperature, is not ductile and can not be rolled to a foil in a satisfactory manner and upon brazing contains a brittle dispersed phase.

Other titanium containing alloys containing higher amounts titanium are known to wet ceramics. These can be made into a ductile foil form by rapid solidification techniques, however, Upon brazing will form a joint containing a brittle dispersed phase.

SUMMARY OF THE INVENTION

Reactive metal-silver alloys containing specified amounts of silver and a reactive metal and controlled amounts of copper or aluminum or mixtures thereof also have liquidus temperatures in the range of from about 750° C. to about 950° C. are ductile and after brazing are relatively free of hard dispersed phases.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE shows the effect upon the peel strength on 99.6% alumina of increasing the titanium content in the 72% Ag 28% Cu eutectic.

DETAILS OF THE PREFERRED EMBODIMENTS

For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims in connection with the above description of some of the aspects of the invention.

Within the context of this application the term "reactive metal" means titanium, vanadium, zirconium and mixtures thereof.

The amount of the reactive metal in a reactive-silver alloy containing copper thereof can not appreciably exceed 4% by weight and achieve a ductile material which upon brazing is free of dispersed phases. Table I below shows the effect upon hardness and ductility of various Ti-Ag-Cu alloys.

                  TABLE I                                                          ______________________________________                                                                       Knoop                                            % Titanium                                                                             % Ag       % Cu       Hardness                                         (by weight)                                                                            (by weight)                                                                               (by weight)                                                                               50 g    Ductility                                ______________________________________                                         0       72         28          80     ductile                                  .5      71.6       27.9       140     ductile                                  1.0     71.3       27.7       145     ductile                                  3.0     69.8       27.2       140     ductile                                  4.0     69.1       26.9       130     ductile                                  4.5     68.8       26.7       201     ductile                                                                201(M)  cracks                                                                 664(D)  2 phases                                 5.0     68.4       26.6       190(M)  cracks                                                                 530(D)  2 phases                                 ______________________________________                                          M = Matrix                                                                     D = Dispersed Phase                                                      

As can be seen from the data in Table I about 4% by weight of titanium is the upper weight limit on the amount of titanium that can be present in the Ti-Ag-Cu alloy system and achieve ductility. Similar results upon ductility are achieved when the Ag-Cu ratio changes to the 72% Ag - 28% Cu eutectic. Additionally similar ductility results are achieved in the Ti-Ag-Al system.

When vanadium or zirconium or mixtures thereof is the reactive metal the material is not as ductile as when titanium is the reactive metal. The presence of brittle dispersed phases relates to the level of the reactive metal and is independent of the particular reactive metal used. When the reactive metal content is above about 2% by weight when vanadium, zirconium or mixtures thereof are used, rapid solidification manufacture technigues or intermediate annealing is required to produce a ductile foil. Titanium is the preferred reactive metal.

In many brazing applications it is desired to utilize relatively low brazing temperatures, that is the liquidus of the alloy should be in the 750° C. to 950° C. range thus the lower brazing temperatures can be utilized.

While the liquidus temperature is in the ranges specified, the flow temperature, that is the temperature at which the alloys of this invention flow and wet the materials to be brazed is important in utilizing the materials using conventional brazing techniques. The flow temperature can exceed the liquidus temperature.

The amount of reactive metal in the alloys is from about 0.25% by weight to about 4% by weight with from about 0.25% by weight to about 2.5% by weight being especially preferred. The amount of copper is from about 2% by weight to about 38% by weight, balance silver.

In most instances the ceramics are ground or ground and lapped prior to brazing. It has been found that for ground and resintered 97.6% alumina ceramic, that the peel strength peaks with a composition having about 2.5% by weight of titanium, about 70.2% by weight of silver and about 27.3% by weight of copper. With ground 97.6% alumina the peel strength drops dramatically with titanium contents higher than about 2.5% by weight. The FIGURE clearly shows the effect of higher titanium contents upon the peel strength of the braze. As it clearly seen about 2.5% titanium is critical when brazing 97.6% alumina which has had conventional surface treatment, that is by grinding or grinding plus sintering or grinding plus lapping. While unground 97.6% alumina does not indicate the criticality of 2.5% titanium, a supplier of the alloy can not be certain of the surface finish of the customer, therefore the brazing foils of this invention containing from about 1 to about 2.5% titanium, balance the levels of silver and copper herein specified have superior overall performance.

To more fully illustrate the subject invention the following detailed examples are presented. All parts, percentages and proportions are by weight unless otherwise indicated.

EXAMPLE 1

Four alloys containing about 1%, 2%, 3% and 4% titanium are prepared using skull melting equipment and a base alloy of the 72% silver, 28% copper eutectic. These four alloys are rolled down to a workable, ductile, 5 mil thick foil.

A portion of each of the foils is placed between two pieces of 97% alumina ceramic which are 3/4" wide by 11/2" long by 0.025" thick. These four samples are brazed at about 920° C. at 10⁻⁵ mm Hg for about 5 minutes. Suitable brazed joints are achieved with all four alloys.

EXAMPLE 2

Silicon nitride sold under the tradename of SNW-1000 by the Wesgo Division of GTE Products Corporation, Belmont, Calif. is satisfactorily brazed to mild steel at a brazing temperature of about 860° C. at 10⁻⁵ mm Hg pressure using a brazing foil of about 3 mils thickness and having composition of about 2% titanium, about 27.4% copper and about 70.6% silver.

EXAMPLE 3

The following alloys are prepared as in Example 1.

    ______________________________________                                         Ag:Cu ratio of 72:28                                                                        99.0%   98.0%     97.0% 96.0%                                     Ti           0.5%    1.0%      1.5%  2.0%                                      Zr           0.5%    1.0%      1.5%  2.0%                                      ______________________________________                                    

The rolled foil is placed between two alumina substrates and brazed. The use of Ti Zr 1:1 ratio shows reduced flow temperature compared to Ti alloy and flows at 840° C.

EXAMPLE 4

An alloy of 70% Ag, 28% Cu, 1.5% Ti, 0.5% V is cast and the ingot section is polished. The observed surface shows no large hard dispersed phases.

EXAMPLE 5

A simple ternary system of AgCuTi which simplifies both manufacturing and analyses were selected over quarternary system. The following alloys are melted and cast into 1/4" thick copper mold. A section of each ingot is cut, polished and observed for dispersed phases.

    ______________________________________                                         SILVER-COPPER TITANIUM FILLER METAL                                            In Weight Percent                                                              A        B      C        D    E      F    G                                    ______________________________________                                         Ag  70%      69%    68%    67%  66%    65%  64%                                Cu  28%      29%    30%    31%  32%    33%  34%                                Ti   2%       2%     2%     2%   2%     2%   2%                                ______________________________________                                    

Photomicrograph of as cast and published specimen are made on all the castings and hardness measurements are also carried out. Differential thermal analyses are carried out on each alloy to determine the solidus and the liquidus temperatures. All ingots are easily rolled down to 2 mil thickness with composition E F & G giving the best finished foils.

    ______________________________________                                         Test Result                                                                    Composition                                                                            A      B      C    D    E      F      G                                ______________________________________                                         Dispersed                                                                              Yes    Yes    Yes  Yes  V. Slight                                                                             V. Slight                                                                             No                               Phase                                                                          Acicular                                                                               No     No     No   No   Yes    Yes    Yes                              Phase                                                                          Knoop   145    142         140  121    125    112                              Hardness                                                                       Liquidus       800         802  804           812                              C.                                                                             Solidus        783         780  775           782                              C.                                                                             ______________________________________                                    

Additionally, it has been found that compositions containing from about 64% to about 66% silver, about 2% titanium and balance copper form braze joints virtually free of dispersed phases, however, it is found that when the percent silver increases and copper decreases so that the silver content ranges from about 67% to about 70% silver, about 2% titanium there is a presence of dispersed phases. It has further been found however that these dispersed phases can be eliminated while maintaining the remaining beneficial properties by substituting vanadium for up to 1/2 of the titanium.

In the foregoing Ti-Ag-Cu system those alloy containing from about 1.75% about 2.5% titanium are especially preferred.

EXAMPLE 6

Peel tests and hermeticity tests show the following compositions to be acceptable for vacuum tube application.

    ______________________________________                                                     Composition in Weight Percent                                                  G     H       I       J     K                                      ______________________________________                                         Ag            64%     63%     62%   61%   60%                                  Cu            34%     35%     36%   37%   38%                                  Ti             2%      2%      2%    2%    2%                                  Liquidus °C.                                                                          812     816     817   818   825                                  Solidus °C.                                                                           780     780     780   780   780                                  Knoop Hardness                                                                               112     112           118                                        ______________________________________                                    

The evaluation of ease to produce foil from large ingot shows that a nominal composition of 63% silver, 35.25% copper, 1.75% titanium is preferred for production.

The preferred composition limits are listed below:

    ______________________________________                                                   Nominal                                                                               Specification                                                 ______________________________________                                         Ag          63.00     62-64%                                                   Cu          35.25    Balance                                                   Ti          1.75     1.50-2.0                                                  ______________________________________                                    

EXAMPLE 7

A peel test is carried out using above alloy foil which is preferred for production namely the 63% Ag, 35.25% Cu, 1.75% Ti material, 4 mil thick, which is placed between 1/4" wide by 10 mil thickness by 3" long Kovar strip and 1.1"×1.1"×0.080" thickness alumina substrate. The assemblies are brazed at the temperatures of 825°, 850° and 875° C. under 10⁻⁵ Torr vacuum. The peel test results show 20 to 25 lb peel strength. The examination of Kovar strip after peeling shows that the ceramic grains are bonded to the filler metal thus indicating excellent bonding.

EXAMPLE 8

Two alumina right cylinders with dimensions of 0.75", outside diameter by 0.25", inside diameter by 1" long are joined to a 0.020" thickness by 0.75" diameter copper disc. A preform of 63% Ag, 35% Cu, 2% Ti with a dimension of 4 mil thickness by 0.75" is placed on each side of the copper disc. The assembly is brazed at 850° C. under 10⁻⁵ Torr vacuum. The brazed assembly is checked for hermeticity using helium mass spectrograph. A leak rate less than 10⁻⁹ cc/sec., which is an acceptable hermetic joint for vacuum tube industry is shown.

Other preferred compositions are those containing from about 1% to about 1.75% by weight titanium, from about 0.5% to about 0.9% by weight of vanadium, from about 64% to about 72% by weight of silver, balance copper.

While there has been shown and described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein with out departing from the scope of the invention as defined by the appended claims. 

What is claimed is:
 1. An article consisting of a ductile foil of a brazing alloy having a liquidus temperature in the range of about 750° to 950° C. and which upon brazing is free of dispersed phases and consists essentially of from about 1.5 to about 2.0% by weight of titanium, from about 62% to about 66% by silver and from about 32% to about 36.5% by weight of copper.
 2. An article consisting of a ductile foil of a brazing alloy having a liquidus temperature in the range of about 750° to 950° C. and which upon brazing is free of dispersed phases and consists essentially of from about 1.5 to about 2.0% by weight of titanium, from about 62% to about 64% by silver and from about 34% to about 36.5% by weight of copper.
 3. An article consisting of a ductile foil of a brazing alloy having a liquidus temperature in the range of about 750° to 950° C. and which upon brazing is free of dispersed phases and consists essentially of from about 0.5 to about 2 by weight of titanium, from about 0.5 to about 2% by weight of zirconium, and the balance of silver and copper, being present in a silver: copper weight percent ratio of 72:28. 